1. Field of the Invention
The present invention relates to a lithographic plate which does not require dampening water (hereinafter referred to as "water-less plate") and, more particularly, to a photosensitive lithographic plate which does not require dampening water (hereinafter referred to as "water-less PS plate"), which is practically useful, for example, in excellent sensitivity, developability and ink-receptive property, as well as in plate-examination property after the development.
2. Description of the Prior Art
Water-less PS plates have been known in which a photosensitive layer and a silicone rubber layer are formed in sequence on a support. Such water-less PS plates are disclosed, for example, in U.S. Pat. Nos. 3,511,178; 3,677,178; 3,894,873 and 4,342,820; and British Pat. No. 1,419,643.
Broadly, there are two methods of preparing a lithographic plate from a water-less PS plate. In one method, the portion of the photosensitive layer corresponding to an ink-receptive area (hereinafter referred to as "image area") is dissolved in a developing solution thereby to remove the portion of the silicone rubber layer on the dissolved portion of the photosensitive layer, thereby to form the image area, as disclosed in U.S. Pat. No. 3,511,178. The other method makes use of the photo-bondability and photo-releasability of the photosensitive layer to selectively remove the portions of the silicone rubber layer corresponding to the image area as disclosed in U.S. Pat. Nos. 3,894,873 and 4,342,820.
The former method is advantageous in that the developability is not substantially affected even when the strength of bonding between the silicone rubber layer and the photosensitive layer is strong enough, because the image area is formed by removing the photosensitive layer. This means that, according to this method, it is possible to obtain a plate having improved anti-scratch and anti-wear characteristics without substantially impairing the developability. On the other hand, this method in which the photosensitive layer is removed by dissolution requires that the photosensitive layer be made as thin as possible. Namely, since the image area is formed by a recess penetrating both the photosensitive layer and the silicone layer, the depth of the recess, i.e. the depth of the image area, will become large when the photosensitive layer has a large thickness. Such a large depth of the image area will require a large amount of ink to be supplied to the plate surface in order to fill such an image area of the large depth.
Under these circumstances, the present inventors have developed a water-less PS plate comprising a primer layer, a photosensitive layer and a silicone rubber layer which are formed in this order on the support. In this improved water-less PS plate, the photosensitive layer has a reduced thickness so that it can operate with a smaller amount of ink than the conventional water-less PS plate. In addition, this improved water-less PS plate exhibits superior anti-scratch and anti-wear chracteristics. The present applicant has already filed applications for a patent on this improved water-less PS plate (Japanese Patent Unexamined Published Application (hereinafter referred to as "J.P. KOKAI") Nos. 60-229031 and 62-50760).
FIG. 1 is an enlarged sectional view of a water-less PS plate wherein primer layer 2, photosensitive layer 3 and silicone rubber layer 4 are formed in this order on support 1.
FIG. 2 is an enlarged sectional view of a water-less plate obtained by exposing and developing the water-less PS plate shown in FIG. 1, wherein image area 4A is formed by a recess which penetrates both the silicone rubber layer and the photosensitive layer and through which the surface of the primer layer 2 can be seen.
A resistance to organic solvents (hereinafter referred to as "solvent resistance") is required for the primer layer. Namely, the primer layer should not be dissolved in or peeled off by an organic solvent used for formation of the photosensitive layer and silicone rubber layer thereon or by a developing solution containing an organic solvent. Further, and excellent dyeability is also required for the primer layer, since the primer layer exposed by the development (i.e. image area) is dyed to facilitate the examination of the plate after the development.
Although the primer layer formed by photo-hardening the photo-dimerization type photo-hardening resin layer as described in the above-mentioned J.P. KOKAI No. 60-229031 has an excellent solvent resistance, it has a defect that the dyeability thereof is poor. The primer layer comprising the hardened epoxy resin described in the above-mentioned J.P. KOKAI No. 62-50760 has defects that when it is hardened sufficiently for imparting the solvent resistance, its dyeability is reduced and, on the contrary, when its degree of hardening is reduced so as to keep the dyeability thereof, its solvent resistance becomes insufficient. It is thus difficult to obtain a water-less PS plate containing a primer layer possessing a good solvent resistance as well as a good dyeability.